In the shoe press apparatus shown in FIG. 6, which is provided in the press part of a papermaking machine, a pair of felts F, and an air-impermeable, endless, elastic belt B, are pinched in a nip N formed by a press roll P and a shoe S. When the press roll P rotates in the direction of arrow P′, the elastic belt B also rotates in the direction of arrow B′, and, as a wet paper web W passes through the press part it is pinched by felts F, and water is squeezed from it. Oil is supplied to the inside of the elastic belt B to reduce friction between the belt and the shoe S.
The surface of the shoe S which is opposed to the press roll P conforms to the shape of the outer surface of the press roll. Therefore, the area of the nip can be much larger than the corresponding area in a press apparatus comprising a pair of press rolls (not shown), and a greater water squeezing effect can be achieved. Therefore, the shoe press apparatus has an important advantage in that much less heat is needed for drying the wet paper web W after water is squeezed from it, and accordingly a significant saving in fuel or energy cost can be realized.
As shown in FIG. 7, which is an enlarged cross-sectional view showing the structure of the elastic belt B, the belt comprises a base member b, and high molecular weight elastic members e on both sides of the base member. The base member b, which is preferably a woven fabric having a warp and weft, is provided to impart strength to the elastic belt B as a whole.
The high molecular weight elastic members e are preferably composed of resin such as a urethane resin, having a Shore hardness A of 70 to 98 degrees. The felt contacting surface and the shoe contacting surface of the elastic belt are both composed of such resins.
A plurality of grooves may be formed on the felt-contacting surface of the elastic belt B, so that the water squeezed from the wet paper web W may be held in the grooves.
Compressed air may be supplied to the inside of an elastic belt B to expand the belt to the shape as shown in FIG. 6.
In the nip N, part of the water squeezed from the wet paper web W moves to the elastic belt B through the felt F which moves between the web W and the belt B. Although most of the water which moves through the felt F to the belt B is shaken off in the direction of arrow a in FIG. 6 as a result of the movement of the belt, part of the water will continue to adhere to the elastic belt B and reenter the nip. Thus, when water adheres to the elastic belt B, the water squeezing effect of the press part may not be adequate.
Therefore, it is conceivable that a doctor blade, similar to the doctor blade used to remove water from a press roll, might be brought into contact with the elastic belt B to remove water from the belt.
Doctor blades used in with press rolls include metallic doctor blades, and doctor blades composed of a felt impregnated with a wear-resistant rubber, synthetic resin or the like, as disclosed in Unexamined Japanese Patent Publication No. 20697/1981. Although such doctor blades are effective to remove excess water from a press roll, problems are encountered in attempts to use such doctor blades to remove water from elastic belts.
A metallic doctor blade can efficiently remove water from an elastic belt, but causes the elastic belt to wear out rapidly. There is also a risk of damage caused by digging of the tip of the metallic doctor blade into the belt. Moreover, the expansion of the elastic belt by compressed air results in a bulging of the belt, such that its outer surface departs from a cylindrical shape. Accordingly, the outer surface of the belt is not necessarily straight in the cross machine direction, and it is difficult to make a metallic doctor blade contact the surface of the belt uniformly.
On the other hand, a doctor blade composed of a felt impregnated with a wear-resistant synthetic resin exhibits excellent adhesion to the surface of an elastic belt, and may have an effect of removing water on the surface of an elastic belt. However, when a resin-impregnated felt doctor blade is used with an elastic belt having grooves, water in the grooves may not be adequately removed because the fibers of the doctor blade may adequately get into the grooves of the belt.